Process for production of metal powders having high green strength

ABSTRACT

Metal particles of high green strength are made by the process of thermally agglomerating a mass of finely atomized particulates of said metal into a self-supporting cake, then milling said cake into a body of agglomerate particles having average particle size substantially greater than said atomized particles. Water or gas atomization can be used, and the metals can be ferrous or nonferrous. The product is useful in powder metallurgy.

United States Patent Klar et all. June 10, 1975 1 PROCESS FOR PRODUCTIONOF METAL 3,479,180 11/1969 Lambert 75/213 POWDERS HAVING HIGH GREEN3,482,963 12/1969 Osborn r 75/0.5 B 3,539,334 11/1970 Goedde1.... 75/0.5B STRENGTH 3,647,417 3/1972 Wetzel 75/5 Ilnventors: Erhard Klar,Baltimore, Md.; Elbert K. Weaver, Westborough, Mass.

Assignee: SCM Corporation, Cleveland, Ohio Filed: Apr. 23, 1973 Appl.No.: 353,659

Related US. Application Data Continuation of Ser. No. 102,682, Dec. 30,1970, abandoned.

US. Cl 75/0.5 B; 29/420; 75/200 Int. Cl B22f 9/00 Field of Search 75/05B, 5, 200, 211,

References Cited UNITED STATES PATENTS 10/1969 Patrick 264/118 PrimaryExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. RosenbergAttorney, Agent, or FirmRichard H. Thomas [5 7] ABSTRACT Metal particlesof high green strength are made by the process of thermallyagglomerating a mass of finely atomized particulates of said metal intoa selfsupporting cake, then milling said cake into a body of agglomerateparticles having average particle size substantially greater than saidatomized particles. Water or gas atomization can be used, and the metalscan be ferrous or non-ferrous. The product is useful in powdermetallurgy.

16 Claims, 2 Drawing Figures PATENTEUJUK'IQ 1975 3,888,857

awe/MM ERHARD KLAR EL BERT K. W EAVER W wW W PROCESS FUR PRODUCTION OFMETAL POWDERS HAVING HIGH GREEN STRENGTH This is a continuation ofapplication Ser. No. 102,682, tiled Dec. 30, 1970, and now abandoned.

This invention relates to a process for producing metal particles havingdesirably high green strength and more particularly to such processwherein the product metal particles are generated from conventionallyfinely-atomized particulates of metal.

Heretofore, it has been proposed to obtain steel particles of high greenstrength by a process of water atomization wherein irregular chain-likeor clump-like atomized particles were produced, which interlocked wellat the pressures used for powder metallurgy and yielded very good greenstrength (U.S. Pat. No. 3,325 .277 While such process is an excellentand economic one, it is restricted as to formation of chain-like orclump-like agglomerates of steel under a narrow range of processingconditions and is oflimited versatility.

It has also been proposed to prepare various metal powders optionallywith additives such as selenium, tellurium, and other additives to makeirregular particles which compact well under powder metallurgy pressuresgive articles of desirably high green strength (U.S. Pat. No.3,383,198). However, for metals other than steel by the process of U.S.Pat. No. 3,325,277, and even for steel using more conventional gas orwater atomization processes, it has been generally recognized in thepowder metallurgy art that one conveniently cannot make desirably highgreen strength metal particles by atomization.

Advantages of our process over prior proposals include versatility withrespect to metals and particle size control thereof, high purity ofmetal where necessary or desirable because additives are not required,control of flowability of the resulting particles and control of purity,e.g. oxygen content or insolubles such as refractory oxides.

The instant process comprises thermally agglomerating a loose mass ofdiscrete finely-atomized particulates of the metal in process into aself-supporting cake, and milling said cake into a body of agglomeratedparticles having average particle size substantially greater than saidoriginal atomized particulates.

The drawings are reproductions of scanning electron micrograph pictures(S.E.M.) showing the unique topology of agglomerate metal particles madein accordance with the principles of this invention.

FIG. 1 shows such particles thermally agglomerated from discretegas-atomized copper particulates and milled, said particles being under175 power magnification.

FIG. 2 shows inventive agglomerate metal particles thermallyagglomerated from discrete water-atomized copper particulates saidparticles being under about 1,500 power magnification. The preparationof these unique particles is described more fully in the examples whichfollow.

In general, the atomized particulates can be formed by conventional gasatomization or liquid atomization processes. Advantageously, forefficiency and economy, water atomization is employed. A leading patentin liquid atomization is the Batten U.S. Pat. No. 2,956,304, for makingsuch atomized particulates.

Other pertinent U.S. Pat. Nos. in the gas atomization art include2,968,062 and 3,253,783.

The starting atomized particulates for our thermal agglomerationtreatment form a loose mass, e.g. a bed, pile, or similar assembly. Forpractical operation, the major portion of such mass should be finer than100 mesh, and advantageously finer than 325 mesh with only a traceexceeding 100 mesh size, (Tyler Standard Sieve). If the major part ofthe starting particulates is substantially larger than 100 mesh, thegreen strength of the resulting product agglomerate particles issubstantially reduced. Preferably, the starting atomized particulatesare finer than about 400 mesh, with only a trace thereof exceeding 100mesh in size. Generally, the proportion of product particles finer than325 mesh does not substantially exceed 30 percent, and product particlescoarser than about mesh are present in trace amounts only.

While not intending to be bound by any theory, it is our belief that theporosity and peculiar topology of the particles resulting from theinventive process makes for their good green strength. Additionally,this process affords excellent independent control over the atomizin gand the agglomerating rather than forcing compromise of any aspects ofone operation for the sake of the other.

Thus, one can visualize a single ideally smooth, spherical, metalparticulate of, say 0.1 inch diameter, having no pore volume whatsoever,accordingly, a pore volume-to-weight ratio of zero. However, when aninfinite number of these are joined together at points of contact(agglomerated) in a gross mass, the pore volume of the intersticesbetween such spheres reaches a constant limiting value. Unfortunately,such mass is too large to handle well and to pour into molds.

Now, we have discovered that by selecting a mass of small enoughatomized particulates, thermally agglomerating them into aself-supporting cake, then braking the cake into small agglomerateparticles which are substantially larger in average mesh size than thatof the original particulates, e.g. up to seven times larger,advantageously about 1.5 to 7, and preferably 3 to 7 times larger, themaximum ratio of the pore volume to weight of an individual agglomerateparticle can be approached in a practical way to yield a product ofuseful, pourable size for powder metallurgy.

We have further discovered that the green strength of the product takesa steep rise from a low value to a desirably high one when such maximumratio is approached in accordance with our processing. The most dramaticgreen strength obtained is for milled cakes having original particulatemakeup of 400 mesh (with only a trace on mesh) milled to a resultingagglomerate particle size of to 100 mesh (with only a trace on 100mesh). Lesser, but valuable, green strength can be obtained for milledcakes having original particulate makeup of 100 to 140 mesh milled to 80to 100 mesh resulting agglomerate particle size. Table III, below,illustrates this observation.

The green strength of conventional compacting grade metal powdersdecreases sharply if the powder is densified by grinding and milling togreater apparent density than its original density. In contrast, powdersmade by the technique described herein are remarkably resistant towardssuch deterioration from grinding or milling. This behavior is of greatpractical significanoe, since it permits an independent control over theapparent density, which in turn is useful to control the flow anddie-filling depth of the powder.

To preserve purity ofthe particles, the thermal agglomeration isdonepreferentially under conventional non-oxidizing conditions, e.g. inthe presence of-hydrogen, carbon 'monoxide, nitrogen, mixtures thereof,using. gas, electric resistance or induction heat, and

generally in an atmosphere containing no reactable ox- .ygen. However,if oxidation can be tolerated, such nonoxidizing conditionswould notnecessarily. be used in thethermal agglomeration or in the optionalannealing Preparatory or during the thermal agglomeration, it

is generally'undesirable to physically compact the atomized particulatestoo greatly. Advantageously, one

' piles them into a loose mass of'about 2-6 inch'thickness withoutvibration, extraneous pressure or other induced compaction. The beddepth, e.g. in a boat, on a belt, or in a sagger, often will be'dictatedby the structural strength of equipment. The agglomeration is done attemperatures of approximately one-half the absolute melting pointtemperature of the metal in process up to a temperature approaching themelting pointof the atomized particulates for times up to about 30minutes, e.g. a few minutes to about 30 minutes, usually at least about5 minutes. A form of sintering takes place in the thermal agglomerationstep of the atomized particulates. The agglomeration can be donecontinuously or batchwise.

Ordinarily the sintered and milled product from our process ischaracterized by a low apparent density, e.g. below 50 percent of thereal density of the metal from which it is made.

The cooled cake can be milled by conventional means, e.g. hammermill,disc mill, fluid energy mill or the like, and if necessary,classification procedures such as screen classification can be used onthe resulting product to obtain especially desirable fractions. Thefollowing examples show how this invention has been practiced, butshould not be construed as limiting the invention. Green strength ofatomized particulates and product particles is measured in accordancewith ASTM B3l2-64 except that a constant compacting pressure of 12 tsiis used. Particle size distribution recited throughout thespecification, examples and claims are US. Sieves Equivalent (FineSieves). It is understood that Tyler Standard Screen Scale Sieves andUS. Sieves can be used interchangeably.

EXAMPLE 1 Seventeen pounds of copper were melted in a kw Ajax inductionfurnace. The melting was done in a graphite crucible. Molten copper waswater-atomized in accordance with the principles of the Batten US. Pat.No. 2,956,304 at a rate of 40 pounds of metal per minute under a waterpressure of 2000 psi and a water velocity of 400 feet per second. Theorifice of the tundish feeding zone measured 3/16 inch. The yield was 92percent finer than 100 mesh and 55 percent finer than v 325 mesh.Thermal agglomeration was done in a laboratory tube furnace with theseatomized copper partic ulates loosely piled in a boat to a bed depth ofl /inches. Temperature of thermal agglomeration was 1'500F under H gasatmosphere, and total timethereof was about 10 minutes. The resultingselfsupporting porous cake was hammermilled into'product agglomerateparticles virtually all but a trace passing mesh,'thesize distributionbeing'about 14 percent finer than 325'mesh, '20 percent between 200 and325 mesh, 12 percent between 80 and mesh, and

the balance between 100 and 200 mesh. This powder had green strength of1380 psi. In contrast, startingat omized particulates, has a greenstrength of psi 5 when tested the same way.

EXAMPLES 2-5 Examples 25'are shown in Table'l. The same steps werefollowed as inExample 1, except that the proportion of finer than 325mesh particulates in the mass thereof subjected to thermal agglomerationranged from 37 percent to 72 percent, with all but a trace passing l00.mesh. The porous cake formed was milled to v virtually all but a tracevpassing 100 mesh, the size distribution being about :20 percent finerthan 325, 30 per% cent between'200 and 325 mesh, and the rest-between100 and 200 mesh.

7 TABLE I Example No. '2 3 4 5 --325 mesh particulates in feed, 63% 69%72% Thermal agglomeration temperature, "F Time, minutes AtmosphereApparent Density g/cc. according to ASTM 8212-48 Flow (sec/50g)according to ASTM B213-48 H Loss according to ASTM El59-63T HNOlnsolubles according to ASTM El94-62T Density of compacted samplesaccording to ASTM B33l63T Green strength of atomized startingparticulates, (psi) Green strength of agglomerate product particles,(psi) Compacts for green strength testing were made with the addition of0.5 percent stearic acid in Examples 3, 4, and 5, and with no additivein Example 2.

EXAMPLES 69 Samples of the product agglomerate particles of theseExamples 6-9 were compacted with 0.5 percent stearic acid, then sinteredfor minutes at 1500F, then compared to compacts made in like manner froma conventional compacting grade of copper powder made by reducing copperoxide.

The foregoing illustrates some of the desirable properties of theinstant process, for example: adaptability of the metal for powdermetallurgy; good green strength, high purity, high sintered strengthboth with and without tin, and excellent compressibility.

EXAMPLE 10 To ascertain the relationship between starting particulatesize and product agglomerate particle size with respect to greenstrength, the following tests were run with nitrogen gas-atomized copperparticulates of three selected screen fractions as individual batches.Each selected fraction was subjected to thermal agglomeration underconditions identical to those set forth in Example One. Each resultingcake was milled to make a mass of product agglomerate particles. Eachmass was classified into fractions of the size ranges indicated in TableIII, Sections A, B, and C. These classified product fractions werecompacted to density of 7.00 grams per cc by the use of the pressureindicated. Compactions for green strength weremade without the use ofadmixed stearic acid.

Example No. 15 7 8 9 Conventional Powder Sintered Properties (PlainCopper) Green Density g/cc 6.32 15.38 6.24 6.33 6.30 CompactingPressure, (tsi) 13.3 114.0 13.3 13.9 16.5 Sint. Density, g/cc 6.1 1 6156.03 6.09 6.33 Sint. Modulus of Rupture, lpsi) 17.500 18,600 16,20014,900 9,400 Dimensional Change 1%) +1.06 +1.29 +1.30 +1.52 +0.47

. TABLE III-A Section B For comparison, the Same conventional compactingStarting Particulates Fraction 10 microns to 400 mesh er owder wasclosed with tin owder and Compacting grade of copp p p Product FractionApparent Den- Green Pressure treated in like manner shown. Samples ofthe product h sity, gms/cc Strength, psi Used, tsi

agglomerate particles of these Examples 6-9 were com- Starting culateswithout pacted with 0.5 percent stearlc acld and 10 percent tin furthertreatment 485 2% 1 5 owder then sintered for 15 minutes in h dro en at325 to 400 mesh 3.32 13 p i y g 200 to 325 mesh 2.70 1860 15.8 1,500F.The tin powder used in all instances had the 140 to 200 mesh 3%: 5828{2.2

t 100 to 140 mesh following characteristics; apparent denslty 3.90 g/cc80 to 00 mesh 5E8 and virtually all but a trace finer than 100 mesh. 50mesh Example No. b 7 8 9 Conventional Powder Sintered Properties(Copperl0% Tin) t g2? Densl y 6.32 6.30 6.31 6.29 6.32 Com actin PFCSSEH'B, (tsi) 111.8 11.5 12.0 11.9 14.2 D 't ens, y 600 5.98 5.97 5.72 5.85 Sint.Modulus of Rupture, lpsi) 29,000 128,900 28,600 24,500 22,600

' 1 Ch i izii ange +2.21 +2.23 +2.05 +2.65 +2.59

Based on 12500" die size.

TABLE III-B Starting Particulates Fraction 325 to 200 mesh StartingParticulates Fraction 140 to 100 mesh Compacting Product FractionApparent Den- Green Pressure Mesh sity, gms/cc Strength, psi Used tsiStarting particulates without further treatment 5.03 85 10.7 80 to 100mesh 340 11.0 50 to 80 mesh 3.78 450 11.4 35 to 50 mesh 3.26 630 12.0 tomesh 2.94 760 12.2

Table 111. A, B, and C show that green strength initially increases withthe increasing size of the product agglomerate particles made from each,then becomes substantually constant and levels off as a critical size ofthe product agglomerate particles is reached. This critical sizedecreases with decreasing size of the starting particulates. Thegreatest gain in green strength results from the agglomeration of veryfine particles.

EXAMPLE 1 l The same steps were followed as in Example 1, except thatthe molten metal was A.I.S.I. grade 304-L stainless steel. The originalatomized stainless steel particulates, when screened, contained 49percent finer than 325 mesh powder. A portion (67 percent) of the -325mesh fraction was thermally agglomerated for 8 minutes at 2100F in an Hatmosphere to form a porous cake. The porous cake was milled tovirtually all but a trace passing 80 mesh and added back the remainderof the unagglomerated particulates from which the 325 mesh was screened.Particle size distribution after agglomeration, milling and blending wasas follows: 6 percent between 80 and 100 mesh, 6 percent between lOO and140 mesh, 18 percent between 140 and 200, 28 percent finer than 325mesh, and the rest between 200 and 325 mesh. The green strength of theresulting particulate mixture measured 1,020 psi when pressed at tsiwith 1 percent lithium stearate admixed as a lubricant. The greenstrength of the starting particulates was 650 psi, when pressed the sameway with the same additive.

What is claimed is:

1. A process for the production of metal particles from finely dividedatomized metal particulates for use in powder metallurgy comprising thesteps of:

a. disposing said particulates in a loose mass and heating said loosemass at a temperature and for a time to produce a porous cake ofminiumum density, said cake being particulates sintered together; and

b. breaking up said cake into particles each having multipleparticulates and having a particle size substantially greater than saidparticulates, said cake being broken up under conditions which minimizedensification.

2. The process of claim 1 wherein said particulates are heated undernonoxidizing conditions.

3. The process of claim 1 wherein said particles have an averageparticle size 1.5 to 7 times greater than the particle size of saidatomized metal particulates.

4 The process of claim 3 wherein a major portion of the atomized metalparticulates is finer than mesh.

5. The process of claim 3 wherein a major portion of the atomized metalparticulates is finer than 325 mesh and only a trace exceeds 100 meshsize.

6. The process of claim 11 further including the step of preparing acompacted metal part from said particles, said compacted metal parthaving substantially higher green strength than if prepared from saidatomized metal particulates.

7. The process of claim 1 wherein the particles are annealed.

8. The process of claim 1 wherein the metal of the metal particulates isnon-ferrous.

9. The process of claim 8 wherein the metal particulates contain copperand heating is carried out at a temperature in the range of about1,000F. to about 1,800F.

10. The process of claim 1 wherein the metal of the metal particulatesis ferrous.

i l. The process of claim 1 wherein the heating is carried out at atemperature of approximately 42 the absolute melting point temperatureof the metal of the metal particulates up to a temperature approachingthe melting point of the particulates.

12. The process of claim 1 characterized in that the apparent density ofthe metal particles is less than about 50% of the real density of themetal of said metal particulates.

13. A process for the production of metal particles from finely dividedatomized metal particulates for use in powder metallurgy comprising thesteps of:

a. disposing said particulates in a loose mass and heating said loosemass at a temperature and for a time to produce a porous cake, saidheating being such as to minimize densification of the mass but beingsufficient whereby said cake is selfsupporting; and

b. breaking up said cake into particles having a particle sizesubstantially greater than said particulates under conditions whichminimize densification.

14. The process of claim 13 including the steps of preparing a compactedmetal part from said particles, said compacted metal part havingsubstantially higher green strength than if prepared from said atomizedmetal particulates.

15. Metal particles thermally agglomerated from dis crete gas-atomizedmetal particulates according to the process of claim 11, said particuleshaving essentially the topology shown in FIG. 1 under about powdermagnification.

116. Metal particles thermally agglomerated from discrete water-atomizedmetal particulates according to the process of claim 11, said particleshaving essentially the topology shown in FIG. 2 under about 1,500 powdermagnification.

1. A PROCESS FOR THE PRODUCTION OF METAL PARTICLES FROM FINELY DIVIDEDATMIZED METAL PARTICULATES FOR USE IN POWDER METALLURGY COMPRISING THESTEPS OF: A. DISPOSING SAID PARTICULATES IN A LOOSE MASS AND HEATINGSAID LOOSE MASS AT A TEMPERATURE AND FOR A TIME TO PRODUCE A POROUS CAKEOF MINIUMUM DENSITY, SAID CAKE BEING PARTICULATES SINTERED TOGETHER; ABDB. BREAKING UP SAID CAKE INTO PRTICLES EACH HAVING MULTIPLE PARTICULATESAND HAVING A PARTICLE SIZE SUBSTANTIALLY GREATER THAN SAID PARTICULATES,SAID CAKE BEING BROKEN UP UNDER CONDITIONS WHICH MINIMIZE DENSIFICATION.2. The process of claim 1 wherein said particulates are heated undernon-oxidizing conditions.
 3. The process of claim 1 wherein saidparticles have an average particle size 1.5 to 7 times greater than theparticle size of said atomized metal particulates.
 4. The process ofclaim 3 wherein a major portion of the atomized metal particulates isfiner than 100 mesh.
 5. The process of claim 3 wherein a major portionof the atomized metal particulates is finer than 325 mesh and only atrace exceeds 100 mesh size.
 6. The process of claim 1 further includingthe step of preparing a compacted metal part from said particles, saidcompacted metal part having substantially higher green strength than ifprepared from said atomized metal particulates.
 7. The process of claim1 wherein the particles are annealed.
 8. The process of claim 1 whereinthe metal of the metal particulates is non-ferrous.
 9. The process ofclaim 8 wherein the metal particulates contain copper and heating iscarried out at a temperature in the range of about 1,000*F. to about1,800*F.
 10. The process of claim 1 wherein the metal of the metalparticulates is ferrous.
 11. The process of claim 1 wherein the heatingis carried out at a temperature of approximately 1/2 THE absolutemelting point temperature of the metal of the metal particulates up to atemperature approaching the melting point of the particulates.
 12. Theprocess of claim 1 characterized in that the apparent density of themetal particles is less than about 50% of the real density of the metalof said metal particulates.
 13. A process for the production of metalparticles from finely divided atomized metal particulates for use inpowder metallurgy comprising the steps of: a. disposing saidparticulates in a loose mass and heating said loose mass at atemperature and for a time to produce a porous cake, said heating beingsuch as to minimize densification of the mass but being sufficientwhereby said cake is self-supporting; and b. breaking up said cake intoparticles having a particle size substantially greater than saidparticulates under conditions which minimize densification.
 14. Theprocess of claim 13 including the steps of preparing a compacted metalpart from said particles, said compacted metal part having substantiallyhigher green strength than if prepared from said atomized metalparticulates.
 15. Metal particles thermally agglomerated from discretegas-atomized metal particulates according to the process of claim 1,said particules having essentially the topology shown in FIG. 1 underabout 175 powder magnification.
 16. Metal particles thermallyagglomerated from discrete water-atomized metal particulates accordingto the process of claim 1, said particles having essentially thetopology shown in FIG. 2 under about 1,500 powder magnification.